This invention relates to antijamming antenna techniques for reducing the vulnerability of line of sight and troposcatter microwave communications links to jamming.
Specifically, the invention comprehends an antenna with an improved gain control circuit for enhancing the null performance of the antenna against a jammer close to antenna boresight.
It has been demonstrated that in existing Defense communications systems there exists the need to protect all directions of arrival of a desired signal against jamming; jamming effectiveness is greatest in the main beam and near-in sidelobe regions of the antenna. Computed jamming/signal ratios for various simulated encounters has showed that an integrated adaptive main beam-sidelobe canceller antenna is required to significantly reduce communication outages due to the various jamming encounters.
Antijamming antenna techniques have been developed which reduce vulnerability of line of sight and troposcatter microwave communications links to jamming. The techniques are incorporated in an antenna system that is configured to adaptively place a null in the direction of a jammer with an arrival angle close to the direction of arrival of a decreased signal (within the main beam) as well as a jammer arriving in the sidelobe region of the main antenna. Spatial discrmination prevents nulling of the desired signal in the absence of jamming an improved signal/jamming ratio for jamming levels comparable to the desired signal level. The antenna system utilized includes an auxiliary antenna that consists of a dual plane, multimode, monopulse feed which replaces the standard feed, a low gain slot array and an omnidirectional antenna.
One problem encountered in the system developed was that of the inability to effectively null jammers that are close to the direction of arrival of the desired signal. In this regard it has been determined, and it is fundamental to the present invention that nulling jammers close to the direction of arrival of the desired signal than is indicated by existing techniques can be achieved at the expense of signal/noise ratio. This represents a trade off that is beneficial for some scenarios.
The method of the present invention of increasing null performance for jammers close to the desired signal DOA is to increase the gain of the monopulse antenna auxiliary difference beams. This can be accomplished spatially or by electronic means. For a specific reflector antenna the aperture size and spatial gain available is already fixed and only electronic gain is available. As indicated above, the penalty for using electronic gain is an increase in the System Noise floor. A particular characteristic of line of sight microwave communication links of the type to which the invention pertains is that they are designated to have large fade margins and therefore the received signal at one of the diversity antennas is usually strong and well above the noise floor. Advantage of this can be taken by the AJ antenna by increasing the electronic gain in the monopulse antenna auxiliary difference beam channels in a controlled manner. This permits nulling a jammer closer to boresight of the antenna. The resulting increase in noise does not degrade system performance in cases where the signal is well above noise floor at one of the diversity antennas.
The invention comprises an antenna system with an improved gain control circuit for enhancing null; performance of the antenna against a jammer close to antenna boresight. The antenna system is provided with an auxiliary monopulse system having sum azimuth difference and elevation difference ports. An adaptive system senses the auxiliary port outputs in conjunction with the signal/noise ratio of the antenna processor output and increases the gain in the difference circuits in response to certain conditions that indicate the presence of a dominant jammer. A dominant jammer is indicated when the power level in the sum port exceeds the power levels of both difference ports and the signal/noise ratio of the processor output is unsatisfactory.
It is a principal object of the invention to provide a new and improved method for effecting main beam nulling in antijamming antennas.
It is another object of the invention to provide an improved method of nulling out interference signals at angles close to boresight in antijamming antennas.
It is another object of the invention to provide, in an antijamming antenna, means for nulling jammers closer to the direction of arrival of desired signals than has heretofore been possible.
It is another object of the invention to provide an antijamming antenna system having adaptive means for detecting the presence of and nulling out jammers close to boresight.
These together with other features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the illustrative embodiment in the accompanying drawings.